The present invention relates to a method of testing the loading capacity of cores. The method according to the invention is especially suitable for testing, for example, the strength of board cores used in the paper industry.
Known cores used in the paper industry comprise several, for example 25-30, thin, e.g. 0.5-1.0 mm, and narrow, about 100 to 200 mm wide, board tapes, which are joined by glueing and spirally winding by a special machine to form a tube-like product, which is used, for example, as a core of paper and plastic rolls.
Known methods for testing cores are almost completely based on static testing. Examples of different types of methods in this group are methods based on definition of radical compression strength, axial compression strength, torsional strength, expansive strength, bending strength and like values. With these methods neither the loading situation nor the loading method corresponds the conditions which the core encounters in reality. Additionally, the break mechanism effected by said types of testing methods differs considerably from the breaks that occur in reality. Such methods are also not capable of finding all defects existing in the core, such as one insufficient glue seam or a weak board layer.
On the other hand, there are also a number of dynamic testing methods for cores, such as utilization of the vibration resulting from rotating the core on a test bench and loading of the core rotating on a chuck with a belt. Although these methods are considerably better than the above mentioned static methods due to their dynamics, even the dynamic methods do not correspond accurately to loading situations. For example, the vibration in the test bench reflects a different characteristic than the loading strength of the core when rolling.
The various methods referred to above do not provide information with sufficient reliability and accuracy about the loading strength of board cores in real rolling situations. Therefore a new type dynamic testing method for cores has been developed. The core is loaded in a manner corresponding the real conditions of center winders and unwinding means of printing presses, whereby the stress exerted on the core by the weight of the paper roll on the chuck can be simulated.
The method according to the present invention for testing the loading capacity of a core or the like member is characterized by mounting the core on a chuck and dynamically loading the core by means of a roll to simulate actual conditions of use wherein the loading of the core and/or the rotational speed are changed relative to time in a predetermined manner until the core breaks; and detecting changes in the core structure and recording the rotational speed and/or the force loading the core at the moment of change and/or the time elapsed at the moment of change.
The following advantages are obtained, for example, by the method and of the invention compared with known methods of testing cores:
a break mechanism as well as break surface end form correspond to real situations;
the method detects even a slightest defect in glueing or board in the core;
good correlation is effected in practice to the roll weights reached by different core qualities;
stress and testing time of the core to be tested correspond better to real life situations; and
the testing apparatus communicates with an automatic indicator of a break point, and there is also direct reporting of the results.